Amplitude modulation produces tremolo at sub-audio rates and carrier±modulator sidebands at audio rates

Amplitude modulation (AM) multiplies a carrier signal by a modulator that varies its amplitude over time. The behaviour depends on modulator rate. At sub-audio rates (roughly below 20 Hz, e.g. 1–10 Hz) the result is tremolo: a periodic pulsing of loudness. At audio rates the multiplication produces sidebands at the carrier plus and minus the modulator frequency (carrier±modulator), giving metallic, bell-like, or vintage-synth timbres. The underlying rule is that multiplication in the time domain equals convolution in the frequency domain, which is why sidebands appear — the same mathematics that underlies AM radio and digital aliasing. Whether the original carrier survives depends on DC offset: an ordinary AM/tremolo modulator carries a DC offset, so the carrier remains alongside the sidebands; remove the DC offset and you get ring modulation, whose output is sidebands only, with the carrier suppressed (at full depth it vanishes entirely). AM uses a single multiplier and is simpler than FM but cannot produce the dense sideband stacks that high FM modulation indices achieve; it is also the basis of amplitude enveloping (multiplying by a shaping function).

Examples

440 Hz carrier × 110 Hz modulator → sidebands at 330 and 550 Hz. A 2–6 Hz modulator on the same carrier → tremolo. Ring mod: 440 Hz × 100 Hz sine with no DC → 340 and 540 Hz only (no 440). Add DC to that modulator and 440 Hz reappears. Pure Data / Bela: [osc~ 440] × ([osc~ 2] + 1) * 0.5.

Assessment

Given a 440 Hz carrier and 110 Hz modulator, predict the sidebands, and contrast a 3 Hz modulator with a 300 Hz one. Explain why AM keeps the carrier in its output while ring modulation does not, and how you shift a ±1 oscillator to 0–1 for use as an AM modulator.